In a conventional electrostatic latent image forming apparatus, as shown in FIGS. 9 and 10, a plurality of stripe-like driving electrodes 51 are provided in parallel to each other on the front surface of an insulating substrate 50, and a plurality of stripe-like control electrodes 52 are provided on the back surface of the insulating substrate 50 so that projections of the control electrodes 52 and projections of the driving electrodes 51 upon a central plane therebetween are mutually intersecting to constitute a matrix. Aperture portions 53 of electrodes 52 are formed at or near those intersections.
Moreover, on the lower surface of the above-mentioned control electrodes 52, as shown in FIGS. 11 and 12, an insulating layer 54 and a screen electrode 55 are provided. In insulating layer 54 and screen electrode 55, as shown in FIG. 11, circular aperture portions 56 and ion derivation aperture portions 58 are aligned with aperture portions 53 of the control electrodes 52.
As shown in FIG. 11, in the above-mentioned electrostatic latent image forming apparatus, a high-frequency high voltage 60 is applied between the driving electrodes 51 and the screen electrode 55, and at the same time, a DC voltage 61 is applied to the screen electrode 55. Moreover, corresponding to picture information, a pulse voltage 62 is applied selectively to the control electrodes 52.
Thus, in the aperture portions 53 between the driving electrodes 51 and the control electrodes 52 across which a voltage is selectively applied, creeping corona discharge R is caused as shown in FIG. 13. An ion current S that is produced by this creeping corona discharge R is accelerated or absorbed by an electric field formed between the control electrodes 52 and the screen electrode 55, and the derivation of ions I is controlled so as to form an electrostatic latent image on a latent image carrying body 59 with the ions I corresponding to a picture signal. In FIGS. 9 to 13, the reference numeral 57 represents a head substrate covering the surface of the driving electrodes 51.
In addition, in such an electrostatic latent image forming apparatus, as disclosed in U.S. Pat. No. 4,160,257, it has been known that the size or shape of recording dots D formed on the latent image carrying body 59 can be controlled by desirably changing the size or shape of the aperture portions 58 of the screen electrode 55, or the distance L between the screen electrode 55 and the latent image carrying body 59.
In the above-mentioned prior art, however, there have been several problems as follows. As shown in FIG. 14, the ions I derived from the aperture portions 58 of the screen electrode 55 electrostatically adhere to the surface of the latent image carrying body 59 so as to form an electrostatic latent image thereon. However, ions I newly derived from the aperture portion 58 of the screen electrode 55 are repelled by ions I already adhering to the surface of the latent image carrying body 59 so that the new ions are spread about. In a recorded picture, therefore, there has been a problem that a picture line is made thick, or, as shown in FIG. 15, adjacent recorded dots D overlap to cause excess image density, that is, the density of a solid picture element becomes excessively high, or spread out, and the resolution is lowered.
If the diameter of the aperture portions 58 of the screen electrode 55 is accordingly reduced as shown in FIG. 16, the expansion of the recorded dots D can be made small (in FIG. 17). In this case, however, the quantity of the ions I derived from the aperture portions 58 of the screen electrode 55 is also reduced, and the charge density of an electrostatic latent image is lowered. This produces a recorded picture having a faint portion or the like.
If the aperture portions 58 of the screen electrode 55 are made small and the distance L between the screen electrode 55 and the latent image carrying body 59 is made small as shown in FIG. 18, a derived electric field acting between the screen electrode 55 and the latent image carrying body 59 increases, so that the quantity of the ions I derived from the aperture portion 58 of the screen electrode 55 increases, and the charge density of a picture element also becomes high.
In this case, however, if there is a pin hole in or dust adhering to a dielectric layer 59b in the latent image carrying body 59 which is formed by coating the surface of a conductive substrate 59a with the dielectric layer 59b, undesirable sparking discharge is caused, as shown in FIG. 18, from the screen electrode 55 supplied with a high voltage toward the latent image carrying body 59, because the distance of the screen electrode 55 from the latent image carrying body 59 is reduced by the dust particle, or the field lines are distorted and concentrated by the pin hole.
As a result, there occurs a problem which brings a failure in the electrostatic latent image forming apparatus or breakdown of the dielectric layer 59b of the latent image carrying body 59.